Roof Truss Modification in Quebec: Costs, Permits & Engineer Requirements

Single-family homes, bungalows, and cottages built in Quebec since the 1960s are very commonly topped with prefabricated roof trusses (also known as engineered roof framing). Designed in a factory using sophisticated structural calculation software, these wood assemblies joined with toothed metal connector plates provide excellent strength while remaining lightweight and cost-effective.

As a family’s needs evolve, homeowners may want to make use of the unused space beneath the roof. Whether the goal is to create a bright cathedral ceiling in the living room, convert the attic into a bedroom, add a dormer, or install a large skylight, modifying the roof’s load-bearing structure requires careful structural planning.

Design snow loads vary considerably from one region of Quebec to another. A roof’s required load capacity is calculated based on local climate data and factors such as roof slope, wind exposure, roof geometry, and potential snow accumulation. Cutting, drilling, or modifying any component of a truss without prior assessment can reduce its structural capacity and compromise the way the system is designed to perform.

Why Modify a Roof Truss, and What Are the Risks?

Source: RenoQuotes

An engineered roof truss does not work the same way as traditional roof framing built piece by piece on site. It is a closed, triangulated system in which each wood component plays a specific and interdependent role under tension and compression forces.

Each component contributes to the overall structural balance:

• Top chord: The sloped wood members that form the roof pitch. They work mainly in compression under the weight of the roofing material and snow.

• Bottom chord: The horizontal member forming the lower portion of the truss, which may support the ceiling below the roof. It works in tension to help prevent the exterior walls from spreading outward.

• Web members or diagonals: The interior pieces forming the triangulated “W” or “V” pattern. They help transfer loads toward the exterior load-bearing walls.

Cutting a web member, chord, or connector can change the way forces are distributed through the truss. Depending on the component affected and the loads involved, this can cause deformation, cracks in interior finishes, and, in more serious situations, compromise the stability of the roof.

Modifying a Roof Truss for an Attic Conversion or Cathedral Ceiling

Homeowners generally consider roof-framing modifications in three common situations.

Attic Conversion

• Goal: Convert unused attic space into a bedroom, office, or playroom.

• Approach: Have a new structural configuration designed to create usable space, potentially using reinforcements, beams, and additional load-bearing elements determined by the structural designer.

Creating a Cathedral Ceiling

• Goal: Alter the flat ceiling to open the space up to the underside of the roof.

• Approach: To create a cathedral ceiling, the existing structure must be recalculated and adapted. Depending on the building, the solution may include a structural ridge beam, additional beams and posts, or replacing certain trusses with a specially designed configuration.

Adding a Dormer or Large Skylight

• Goal: Bring in more natural light and increase usable headroom.

• Approach: Creating an opening may require interrupting certain trusses and redistributing their loads using headers and reinforced adjacent trusses or framing members, according to the structural plans.

Each type of modification can change how roof loads are redistributed through the building’s load-bearing elements. Before planning your interior renovation, it is advisable to request quotes to modify your roof truss and get input from qualified framing specialists.

When Do You Need an Engineer to Modify a Roof Truss?

Source: RenoQuotes

In Quebec, modifying a prefabricated roof truss is not simply a carpentry job. It is regulated by the Engineers Act and the Quebec Construction Code.

Legal Requirements

• A roof-truss modification must be designed so that the roof maintains its structural capacity. Depending on the type of building, the extent of the modification, the proposed solution, and municipal requirements, structural calculations and plans prepared by an engineer may be required.

Manufacturer’s Warranty

• The manufacturer’s warranty may be affected if the truss is modified from its original design.

• Any modification should be designed or validated by an engineer to help ensure structural compliance.

A contractor or carpenter must perform the work in accordance with the applicable structural design. When a design or calculation activity is reserved under the Engineers Act, it must be carried out by an engineer who is a member of the OIQ.

What Is Included in the Engineer’s Report and Sealed Plans?

When you hire an engineer to assess your roof, the process generally involves several steps:

1. Site inspection and existing-condition survey: The engineer inspects the attic to measure lumber sizes, truss spacing usually 16 or 24 inches on centre, the condition of the metal connectors, and the span between load-bearing walls.

2. Structural modelling and load calculations: Using structural calculation software, the engineer assesses how the roof performs under local snow loads, wind loads, and the weight of new materials such as insulation, drywall, or flooring.

3. Preparation of modification plans: When required, the engineer prepares structural plans showing the elements to be modified or reinforced, their dimensions, the materials to be used, and the necessary connections and fasteners.

4. Engineer’s seal: The engineer stamps their official seal and digital signature on the plans. Signed and sealed plans can then be included with the municipal permit application when required. The municipality issues the permit after reviewing the application.

Depending on the plans for the specific project, the work may include temporary shoring of the roof, adding new framing members or beams, reinforcing certain connections, and carefully removing existing components. Dimensions, materials, connectors, and installation sequences must follow the applicable structural plans.

Obtaining a Municipal Renovation Permit in Quebec

Source: Construction Léonard inc.

A structural modification to a roof will generally require a municipal permit. However, the requirements and documents requested vary by municipality, so check with your local planning department before work begins.

Depending on the municipality and the scope of the work, the permit application may include:

• The completed permit application form.

• A certificate of location.

• Plans showing the existing and proposed conditions.

• Structural plans, when required.

• Information about the contractor.

Risks associated with carrying out work without the required municipal permit include:

• The possibility of a municipal stop-work order.

• Fines imposed under applicable municipal bylaws.

• Undeclared or non-compliant work potentially complicating an insurance claim, depending on the circumstances and policy conditions.

• Significant complications when selling the property.

Taking administrative shortcuts with structural work can put your investment at risk. When the property is sold, the seller’s declaration may require you to disclose structural work completed without a permit. This can raise questions during the transaction, require additional inspections, or require the owner to have the work legalized or corrected.

How Much Does It Cost to Modify a Roof Truss in Quebec?

Source: 9253-4650 Québec inc

The total budget for modifying a roof truss generally includes three main components: professional engineering fees, municipal permit fees, and the cost of framing materials and labour.

1. Engineer’s fees (OIQ): $1,000 to $2,500

2. Municipal renovation permit: $100 to $500

3. Framing materials and labour (RBQ contractor): $2,500 to $10,000

Estimated total budget: $3,600 to $13,000+

To better gauge project cost, request a quote from an RBQ-licensed general contractor who can complete the work in compliance with the engineer’s plans.

Engineer’s Fees and Municipal Permit Costs

Engineering fees vary depending on the architectural complexity of the home and whether original building plans are available.

Indicative Cost Ranges by Type of Project

The cost of the work depends heavily on the nature of the project. Modifying two or three roof trusses to add a dormer is significantly less expensive than modifying the entire roof structure to create a cathedral ceiling in an open-concept space.

Indicative framing costs by project type:

Local Modification (Skylight/Dormer)

• Scope: 2 to 4 roof trusses affected.

• Cost of work: $2,500 to $4,500, including materials and labour.

Partial Modification (Limited Attic Conversion)

• Scope: Reinforcing bottom chords and installing gussets over a 15- to 20-foot section.

• Cost of work: $4,500 to $8,000.

Major Transformation (Full Cathedral Ceiling)

• Scope: Major structural reconfiguration involving a structural ridge beam and/or modified trusses, based on the engineer’s design.

• Cost of work: $8,000 to $15,000+.

Material choices also have a significant impact on the final cost:

• Engineered wood beams (LVL/Microllam): Approximately $15 to $35 per linear foot, depending on size.

• Gussets and 3/4-inch plywood: Approximately $45 to $65 per 4 × 8-foot sheet.

• Heavy-duty steel connectors: Approximately $5 to $25 each, depending on load capacity.

You can also request free quotes from qualified carpenters to compare material prices and labour rates.

Choosing an RBQ-Licensed General Contractor

Source: RenoQuotes

Modifying a load-bearing roof structure requires careful execution. Improper handling during temporary shoring can cause ceilings below to crack or allow the roof structure to shift, potentially resulting in water infiltration and other damage.

In Quebec, make sure the company you hire holds a valid licence issued by the Régie du bâtiment du Québec (RBQ) with the appropriate subclassifications:

• Subclass 1.2: Contractor — Small Buildings

• Subclass 1.3: Contractor — Buildings of All Kinds

Checklist for choosing your contractor:

• Verify the RBQ licence number in the public registry.

• Confirm that the contractor carries adequate liability insurance.

• Require the work to be completed in accordance with the engineer’s plans and requirements.

• Verify which contractual and legal warranties apply to the framing work.

Article 2118 of the Civil Code of Québec provides, under certain conditions, for five-year liability related to the loss of the work resulting from a defect in design, construction, or performance, or from a defect in the soil. This legal liability is separate from the contractor’s obligation to hold the required RBQ licences.

To complete your project with greater peace of mind, take some time to find an RBQ-licensed general contractor specializing in residential structural repairs and modifications.